[gpfsug-discuss] Removing LUN from host without unconfiguring GPFS filesystem

[Jonathan Buzzard]

On 22/01/18 03:41, Stephen Ulmer wrote:
> Harold,
> 
> The way I read your question, no one has actually answered it fully:
> 
> You want to put the old file system in cold storage for forensic 
> purposes — exactly as it is. You want the NSDs to go away until and 
> unless you need them in the future.
> 

Hum, I would have said I did answer it fully even if I didn't go into 
detail. The only in my view sensible approach is to do mmexportfs so 
that should you need access to the data then you can get to it by 
reimporting it using mmimportfs. In the interim you can unmap all the 
NSD's from the cluster without causing GPFS to go care in the slightest.

Otherwise you are doing something that is in my view at the best fool 
hardy and more generally down right idiotic. Personally I would outright 
refuse to do it without someone else higher up signing a written 
disclaimer that I was not responsible should it all go pear shaped. Note 
that the mmexportfs takes a few seconds at most to complete, and 
likewise with the mmimport.

I am however somewhat perplexed by the data integrity side of the issue. 
If you suspect the data is corrupt in the old file system then having it 
around for reference purposes is surely not going to help. What are you 
going to do mount it again to find the file is corrupt; how is that 
going to help?

If you suspect that whatever method you used to move the files from the 
old file system to the new one may have introduced corruption, then I 
would suggest that the best way out of that is to do an rsync with the 
-c flag so that it takes an MD5 checksum of the files on both file 
systems. Once that is complete you can safely ditch the old file system 
completely knowing that you have recovered it as best as possible. You 
could probably kick a bunch of rsyncs of in parallel to speed this 
method up.

In fact a "rsync -c" would be a gold standard of look it's absolutely 
the same on the old and new file systems and remove all doubts about the 
transfer introducing corruption. At that point if someone comes and says 
your transfer corrupted my files, you can with justification say they 
where corrupt in the old file system and I have done my best to transfer 
them over. Note if any user was deliberately creating MD5 collisions 
then they get everything they deserve in my view, and accidental 
collisions in MD5 are about as likely as ħ.


JAB.

-- 
Jonathan A. Buzzard                         Tel: +44141-5483420
HPC System Administrator, ARCHIE-WeSt.
University of Strathclyde, John Anderson Building, Glasgow. G4 0NG